Improvements to README.md

[tomasssalles]

There's a lot that we could do better in the README. Here is some feedback from a few beta users about the 0.2.0-beta.1 version:

• It reads more like a blog article than a README.
• It should quickly and succinctly address the simplest and most common issues that the package triesto solve.
• Less details, more use cases.
• Even if the whole content remains in the README, it should at least contain a simpler example, just with a config file, a config.py and a __main__.py, with very little logic.
• Focus a bit more on the use of environment variables, because many users will find it especially useful when working with containers.
• Maybe move some of the explanation of the code snippets into them as comments.